(rail transport) A multiple-unit train of two or more coaches powered by diesel engines, usually installed underfloor.

(rail transport) A multiple unit train powered by electricity, abbreviated EMU.

(rail transport) An electrically driven multiple unit or railcar whose energy is derived from rechargeable batteries driving the traction motors.

An electro-diesel multiple unit or bi-mode multiple unit is a form of a multiple unit train that can be powered either by electric power picked up from the overhead lines or third rail (like an electric multiple unit – EMU) or by using an onboard diesel engine, driving an electric generator, which produces alternating current or direct current electric power (like a diesel-electric multiple unit – DEMU).

A number of different numbering and classification schemes were used for locomotives and multiple units operated by British Railways, and this page explains the principal systems.